The guidance of axons towards their targets is one of the first steps in the development of selective neural connections. The long term aim of the experiments described in this proposal is to determine the cellular and molecular events that underly the guidance of axons and growth cones towards their targets within the mammalian central nervous system. The work focuses on the navigation of the axons of commissural neurons in the spinal cord towards an intermediate target in their trajectory, the floor plate cells at the ventral midline of the spinal cord. The projection of commissural neurons is stereotyped and occurs in two stages, the initial ventral extension being confined to the lateral edge of the spinal cord and the subsequent projection directed medially and ventrally to the midline. This stereotyped pathway may be due to the presence of several distinct guidance cues. The purpose of this proposal is to examine the nature and function of these cues. Preliminary experiments have revealed that the pathway taken during the initial ventral extension of commissural axons is restricted to a region of spinal cord neuroepithelium expressing an extracellular matrix component, recognized by the antibody INO, and probably comprising a laminin-heparan sulphate proteoglycan complex. The role of the extracellular matrix in restricting the growth of commissural axons will be examined in vitro. The extension of commissural axons on the INO complex will be examined. In addition, the ability of antibodies directed against INO to modulate axonal extension will be determined. Finally, the role of receptors on commissural axons for the glycoproteins in the substratum will be examined. We have shown that floor plate cells release a chemoattractant factor that influences the direction of growth of commissural axons in vitro and may be responsiblefor the medial trajectory of commissural axons towards the floor plate in vivo. This provides the first experimental evidencefor the involvement of chemotropism in the developmentof the central nervous system. The chemotropic effect will be examined in detail with emphasis on the time course and range of the action of the tropic factor. In addition, the biochemical properties of the factor will be determined in order to facilitate its purification and the analysis of its mechanism of action. The proposed experiments should enhance our understanding of the nature, regulation and interaction of events leading to the developmental trajectory of one set of central neurons, commissural neurons of the spinal cord. The information will also permit us to determine how many of these mechanisms are likely to be used in other parts of the developing central nervous system. Many of the mechanisms underlying initial axonal extension and the guidance of axonal projections towards appropriate targets may also be essential for regeneration of axons. Understandingthese mechanisms is fundamental to our ability to treat degenerative or developmental disorders of the nervous system.